1. Field of the Invention
The present invention relates to an optical communication network system such as an optical local area network (LAN) and a communication method using the same.
2. Related Background Art
In recent years, optical communication networks such as optical LANs have been remarkably developed. These networks must have compatibility with multi media in future applications. Wavelength multiplexing is most promising as a solution to this problem. Many problems are still left unsolved to realize a wavelength multiplexing LAN compatible with multi media.
First, when a system is to be constituted by terminal stations passive to light, attenuation is repeated by terminal stations, and the number of terminal stations connected to the system is limited to result in restriction to system extensibility.
Repeaters must be arranged in the system to prevent the attenuation. It becomes, however, more difficult to perform amplification by optoelectrical (O/E) conversion and electrooptical (E/O) conversion in units of channels as the degree of wavelength multiplexing is increased. In order to solve this problem, it is regarded as an effective method to use a wide-band optical amplifier for collectively amplifying signals on a plurality of channels.
In a wavelength multiplexing LAN, the input level of the optical amplifier greatly varies depending on the number of channels used for multi-access. It is, therefore, difficult to set the output level to a desired level.
Second, temperature stability of an oscillation wavelength of a semiconductor laser used as a light source is poor. In order to prevent an error rate decrease caused by crosstalk between channels, a channel separation range must be increased. As a result, semiconductor lasers corresponding to the number of channels and narrow-band stable wavelength filters (e.g., dielectric thin film interference filters and diffraction gratings) corresponding to the number of channels are required to constitute a light source, resulting in high cost and limitations the degree of wavelength multiplexing and hence failing to satisfy needs of users.
Third, in association with the second problem, there is provided a method of using a wavelength variable laser as a light source and a semiconductor wavelength variable filter as a filter to increase a degree of wavelength multiplexing. In order to assure the number of common channels for the respective terminal stations, high-precision temperature control is required for the semiconductor laser and the semiconductor wavelength variable filter, resulting in an expensive system.